1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus.
2. Description of the Related Art
As a technique concerning the sheet processing apparatus, for example, Japanese Patent Application Laid-Open No. 2003-95506 discloses a technology relating to a sheet processing apparatus that applies predetermined processing to a sheet-like recording medium (sheet). The sheet processing apparatus includes a staple processing tray that applies alignment and staple processing to a sheet stack, a conveying path that directly discharges the sheet stack subjected to the alignment and staple processing by the staple processing tray, upper and lower stack conveying guides that convey the sheet stack to the side of a folding plate that performs center folding, and a branch guide plate and a movable guide plate that switch a path for conveyance of the sheet stack to the conveying path and the upper and lower stack conveying guides. When the branch guide plate and the movable guide plate switch the path for conveyance of the sheet stack to the upper and lower stack conveying guides, the branch guide plate and the movable guide plate deflect the sheet stack along the outer periphery of a discharging roller located on a most downstream side of the staple processing tray to guide the sheet stack to the upper and lower stack conveying guides.
Japanese Patent Application Laid-Open No. 2003-155155 discloses a technology relating to another sheet processing apparatus that applies predetermined processing to sheets conveyed thereto. The sheet processing apparatus includes a staple processing tray that applies alignment and staple processing to a sheet stack, a conveying path for directly discharges the sheet stack subjected to the alignment and staple processing by the staple processing tray, upper and lower stack conveying guides that convey the sheet stack to the side of a folding plate that performs center folding processing, and a branch guide plate and a movable guide plate that switch a path for conveying the sheet stack to the conveying path and the upper and lower stack conveying guides. The branch guide plate and the movable guide plate are set in any one of a first position for conveying the sheet stack to the conveying path, a second position for conveying the sheet stack to the folding plate side, and a third position in the middle of the first position and the second position. It is possible to perform deflection of the sheet stack more surely by providing the third position.
In a technology disclosed in Japanese Patent Application Laid-Open No. 2000-211795, an end in a direction orthogonal to a sheet conveying direction is aligned by a jogger fence and a trailing end in the sheet conveying direction is aligned by a trailing end fence. A discharging pawl that pushes up a sheet stack on a staple tray and discharges the sheet stack is moved in a counter-discharge direction to bring a rear side of the discharging pawl into contact with a leading end of the sheet stack and perform an operation for aligning the sheet stack.
In a technology disclosed in Japanese Patent Application Laid-Open No. H11-199118, a sheet is received until a leading end of the sheet exceeds a downstream side end according to operation control by a control device in a separated state of a stack discharging roller pair including an upper stack discharging roller and a lower stack discharging roller. The upper stack discharging roller is brought into contact the lower stack discharging roller to hold the received sheet between the stack discharging roller pair. The lower stack discharging roller is reversely rotated to pull back the sheet to an upstream side. The upper stack discharging roller is separated from the lower stack discharging roller in a position where a center of gravity portion of the sheet passes a downstream side end. After the processing is finished, a sheet stack is discharged onto second stacking tray means. In this way, a pull-back action for the sheet-like recording medium during aligning operation is effectively performed to prevent alignment failure. In this conventional technology, when more than half of a discharged sheet is discharged to the second stacking tray, it is difficult to draw the sheet with a drawing paddle that strikes a trailing end of the sheet into contact against a trailing end stopper. Therefore, the discharged sheet is nipped by the stack discharging roller pair, which is a roller pair that discharges a stack, and conveyed in a direction opposite to a sheet conveying direction, and more than half of the sheet is pulled back onto the first stacking tray to strike the trailing end of the sheet against the trailing end stopper and improve an alignment state.
When center binding or folding processing is performed, after sheets are aligned in a conveying direction and a width direction by an end-binding processing unit, a sheet stack is conveyed to a center binding processing unit located downstream to align the sheets in the conveying direction and the width direction again. In that case, it is more advantageous to maintain an aligned state of the sheet stack conveyed to the center binding processing unit as much as possible to improve an alignment quality of the sheet stack after alignment processing and center binding and folding processing to be performed thereafter.
However, in the conventional sheet processing apparatuses disclosed in Japanese Patent Application Laid-Open Nos. 2003-95506 and 2003-155155, the conveying path that conveys the sheet stack from the end-binding processing unit to the center-binding processing unit is bent. When the conveying path is formed in such a shape, since a conveyance resistance applied to the sheet stack increases, a sheet jam tends to occur. Moreover, since a sheet conveyance difference occurs between an inner side and an outer side of a bent section, shift of the sheets occurs in a conveying direction.
When the center binding or folding processing is performed, after the sheets are aligned in the conveying direction and the width direction, conveying unit that applies a conveying force to the sheet stack in conveying the sheet stack to the center binding processing unit located downstream is located above an upper end-binding processing unit. Therefore, it is necessary to prevent the position, control, and the like of the conveying unit from hindering the sheet alignment in the end-binding processing unit. For example, in aligning the sheets in the end-binding processing unit, when the conveying unit located above and a leading end of the sheets entering the end-binding processing unit come into contact with each other, the entrance of the sheets is hindered if the conveying unit operates in a direction opposite to the conveying direction. Moreover, since a force in the direction opposite to the conveying direction is suddenly applied to the sheets, the sheets may be buckled. The conventional technologies disclosed in Japanese Patent Application Laid-Open Nos. 2003-95506, 2000-211795, and H11-199188 cannot cope with such a bend of the sheets.